The problem of which
evolved
first, the lung or the swim bladder, is complicated because their
developmental
origins are similar. Both arise from an outgrowth of the
gastrointestinal
tract.

You might think that the
ancestor
of all fish would need a swim bladder, an organ whose function is to
control
bouncy.

You might also think that
fish,
since they live in water, would have no need for lungs, which are
used
to breathe air.

Ah, if only the world
were
so simple! However, there are fish with lungs and fish without swim
bladders!

There is evidence, based
on
the phylogenetic distribution of the two organs, that lungs are the
more
primitive of the two. But perhaps this is an overly simplistic view.
You
can easily see how a lung could act as a swim bladder (you float
better
when your lungs are inflated) and a swim bladder as a lung (oxygen
in
a inflated sack could be recovered and used for respiration).
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Fish with jaws and fish
capable
of living in fresh water first arose in the Silurian
b>
period. The current consensus (as far as I can make out) is that
earliest
lungs were sacks used to hold air gulped in response to low levels
of
oxygen in stagnant pools.

It is thought that
drought
(and hence fish stranded in stagnant pools) was common in the early
Devonian
period. The development of a "back-up' breathing system would
therefore
have been quite advantageous, even if it worked rather poorly.
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Once developed, the
protolung/gas
sack could be easily modified to improve its efficiency as a gas
exchanger.
This would also make it possible to use regulator of density, i.e.
to
function as an swim bladder.

As Romer and Parsons
(1977)
state, "The swim bladders is found only in one subdivision of
the
bony fishes, the actinopterygians, whereas the most primitive member
of
that group, and members of the Sarcopterygii as well, have a lung.&
quot;